Spray application of refractory fiber is a desirable method of producing a refractory material in furnaces, kilns, and other thermally insulated structures. Advantages of such a method include reduced costs and reduced downtime or the like. That is, as opposed to previous methods for relining furnaces, requiring either manual replacement of brick or mechanically attaching refractory fiber linings, spray insulation may be easily and quickly applied to the surfaces.
A number of spraying techniques or applications have been disclosed or known to those skilled in the art. While they have been satisfactory for their intended uses, these known methods have not been without limitations.
U.S. Pat. Nos. 4,547,403 and 4,673,594 both disclose the use of colloidal silica, colloidal alumina, colloidal zirconia, and aluminum phosphate as binders in conjunction with processes for spraying refractory fibers upon substrate surfaces. The limitation associated with these disclosed processes, though, is that the refractory layer sprayed upon a substrate surface may debond from that substrate upon thermal cycling. That is, while the process may result in bonding of the refractory fiber one to another, the bond between the fiber binder layer and the substrate upon which it is sprayed may be deficient. U.K. patent application Nos. 2,093,014 and 2,093,015 also disclose spraying applications utilizing phosphate bonding which would therefore suffer from the same limitations.
Thus, even though it is recognized that aluminum phosphates, colloidal silica, colloidal alumina, and colloidal zirconia are advantageous to use because of their economics and ready availability, attempts have been made to employ other binders in spraying processes because of the foregoing disclosed limitation. For example, one alternative method for applying refractory insulations utilizes refractory fibers and a hydraulic setting-inorganic binder. This product is marketed under the trademark CERAMOSPRAY.RTM. which is a registered trademark of the U.S. Mineral Products Company. While suited for its intended purpose, this method utilizes a dry cementitious binder which is limited in its temperature range to a stated maximum of 2200.degree. F. In addition, a cementitious binder such as calcium aluminate or calcium phosphate may adversely affect silicate based brick products at temperatures in excess of 2000.degree. F., thus effectively reducing the maximum service temperatures of these cementitious refractory materials in such applications. Mechanical anchors are also required with this method. However, the installation of mechanical anchors is both labor and time intensive.
Still another spray application method is disclosed in Japanese Pat. Nos. 51-40846 and 49-87723. These references disclose the use of dry alumina cement the amount of which must be limited to avoid very high densities thereby affecting the thermal insulating properties of the layer. In addition, as stated in those references, when used in large quantities the scattering of cement dust creates an undesirable pollution problem. While the use of colloidal alumina is disclosed, it is only disclosed with a dry material such as bentonite to give it bonding characteristics. Such a system may suffer from weak bonding at the substrate interface.
Another sprayable ceramic fiber insulation system marketed under the trademark FIBERBRAX.RTM., a trademark of the Sohio Engineered Materials Company (CARBORUNDUM), is a sprayable system which is rated to a maximum temperature of 2100.degree. F. While suited for its intended purpose, the system is limited in temperature application and is also a silica based binder material which is subject to debonding at the substrate interface as with the clay and phosphate binders. To improve the bonding of the layer to the substrate, mechanical anchors are recommended for many applications. As can be appreciated by one skilled in the art, the use of anchors increases cost and furnace down time. Anchors are also unsuitable for a badly deteriorated brick or other substrate since attachment of the anchors to the substrate may be difficult and/or dangerous.
U.S. Pat. No. 4,664,969 discloses a method for applying refractory fiber to a surface employing a liquid binder such as aluminum chloride which is capable of forming alpha alumina (Corundum) crystals upon curing. While suitable for its intended application, the disclosed process suffers from the limitation that upon curing of the binder, hydrogen chloride vapor can be released which can pose a stress-corrosion problem in some instances. For example, in petro-chemical plants where oil is typically piped through stainless steel tubes that are supported by stainless steel hangers, the HCl vapor is corrosive toward the stainless steel. Obviously, the consequences of possible failure due to stress-corrosion of the supporting stainless steel tubes could be great. The evolution of HCl vapor can also pose a potential health hazard to those working in the area.
From the foregoing, it can be seen that attempts to circumvent aluminum phosphate and colloidal refractory particle binding systems have not always resulted in superior processes. Therefore, what is needed in the industry is an application process which employs these binder systems but yet is economical, efficient, and easy to practice and which does not possess the foregoing disclosed limitations.